Light emission circuit

ABSTRACT

Apparatus comprises first and second light sources driven respectively by second and third inverting amplifiers with feedback from the first and second light sources to provide signal content respectively on positive and negative phases of an input signal; and a bias control arrangement configured to measure a bias level of one of the light sources and to bias the second and third amplifiers based on the measured bias level.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of United KingdomPatent Application No. 1313788.0, filed in the United Kingdom PatentOffice on Aug. 1, 2013, the disclosure of which is incorporated hereinby reference.

FIELD OF THE INVENTION

This invention relates to a light emission circuit.

BACKGROUND OF THE INVENTION

In a concert or studio environment, it is usual to connect soundsources, including guitars and microphones, to amplification equipmentwith shielded electrical cables. Such cables have a copper core and ametallic shield separated by an insulating material. At each end, a jackor plug, typically ¼ inch (6.35 mm) diameter, allows both the coppercore and the shielding of the cable to be electrically connected toelectronic circuitry in the relevant electrical equipment. Signals arecarried from the sound source in the same form in which they aregenerated, i.e. as analogue signals in the audible frequency range 20 Hzto 20 kHz. In a studio environment, the same type of cable connectssound sources to mixing desk equipment. Cables used in concertenvironments can be over 10 m long, although shorter cables tend to beused in studio environments.

Such cables can become internally damaged during use, especially whenbeing used in an on-stage environment, although they may appearexternally to be undamaged. Damaged cables cause a reduction in thequality of signals being carried, and may result in unwanted distortionor other degradation of the audio signals. If the core of the cablebecomes fractured, the cable can stop functioning altogether, althoughsignal deterioration is more common. The inventor considers that signaldegradation might result from fractures in the core and/or shieldingand/or from damage to the insulating material separating the core fromthe shielding resulting in unwanted inductances and/or capacitances,which can cause unwanted resonance and/or filtering when supplied withenergy in the form of the audio signals being carried.

The effects of cable damage can be avoided through the use of radiolinks between the sound source and the mixing desk or amplificationequipment. Radio microphones are well known. However, the possibility ofradio interference means that digital communication links are morereliable. However, many musicians and sound producers prefer audiosignals not to be digitised at any point in their transmission on thebasis that this results in a reduction in quality, as well as a lesspure sound.

WO2007/031794 discloses arrangements that mitigate these disadvantages.The present invention builds upon the arrangements disclosed inWO2007/031794.

SUMMARY OF THE INVENTION

A first aspect of the invention provides apparatus comprising:

first and second light sources (LED1, LED2);

a first transistor (Q1) having first and second main electrodes and acontrol electrode;

a second transistor (Q2) having first and second main electrodes and acontrol electrode;

a first amplifier (U1) having first and second inputs and an output;

a second amplifier (U2) having an input and an output;

a third amplifier (U3) having an input and an output; and

a bias control arrangement having an input and an output; wherein:

the first light source (LED1) and the first and second main electrodesof the first transistor (Q1) are connected in series in a path between apositive supply terminal and a first node,

the second light source (LED2) and the first and second main electrodesof the second transistor (Q2) are connected in series in a path betweenthe first node and a negative supply terminal,

the first node is connected to a second input of the first amplifier(U1),

the input of the second amplifier is connected to the output of thefirst amplifier,

the output of the second amplifier is connected to the control input ofthe first amplifier,

the input of the third amplifier is connected to the output of the firstamplifier,

the output of the third amplifier is connected to the control input ofthe second transistor,

the input of the first amplifier is connected to a signal input terminal(INPUT),

the input of the bias control arrangement is connected to a second nodein the path through the first and second light sources and the first andsecond transistors,

the bias control arrangement is configured to measure bias at its inputand provide a control bias signal at its output, and

the output of the bias control arrangement is connected to the input ofeach of the second and third amplifiers.

The output of the bias control arrangement may be connected to the inputof each of the second and third amplifiers via an arrangement comprisinga fourth amplifier (U4) and a buffer (U5). Here, the output of the biascontrol arrangement may be connected to an input of the buffer (U5),wherein an output of the buffer may be connected to the input of thethird amplifier (U3) and to an input of the fourth amplifier (U4), andwherein an output of the fourth amplifier (U4) may be connected to theinput of the second amplifier (U2).

The apparatus may comprise a first resistor connected directly to thefirst node and in the path to the positive supply terminal and a secondresistor connected directly to the first node and in the path to thenegative supply terminal, and wherein the second node may be connectedto the first node by either the first resistor or the second resistor.

The bias control arrangement may include a microprocessor. Themicroprocessor may be configured to monitor signals received at thesignal input terminal and is configured automatically to power down inresponse to detecting no significant input signal for a period of time.

The bias control arrangement may have a second input that is connectedto the input of the first amplifier, and the bias control arrangementmay be configured to calculate the bias control signal based oninstances when the level of the signal at the second input is low and torefrain from calculating the bias control signal based on instances whenthe level of the signal at the second input is not low.

The bias control arrangement may be configured to provide at its outputa pulse width modulated signal having a duty cycle that is a function ofthe measured bias.

A second aspect of the invention provides apparatus comprising:

first and second light sources driven respectively by second and thirdinverting amplifiers with feedback from the first and second lightsources to provide signal content respectively on positive and negativephases of an input signal; and

a bias control arrangement configured to measure a bias level of one ofthe light sources and to bias the second and third amplifiers based onthe measured bias level.

The output of the bias control arrangement may be connected to thesecond and third amplifiers via an arrangement comprising a fourthamplifier and a buffer. Here, the output of the bias control arrangementmay be connected to an input of the buffer, an output of the buffer maybe connected to the input of the third amplifier and to an input of thefourth amplifier, and an output of the fourth amplifier may be connectedto the input of the second amplifier.

The bias control arrangement may include a microprocessor. Themicroprocessor may be configured to monitor the input signal terminaland is configured automatically to power down in response to detectingno significant input signal for a period of time.

The bias control arrangement may be configured to calculate a biascontrol signal provided to the second and third amplifiers based oninstances when the level of the input signal is low and to refrain fromcalculating the bias control signal based on instances when the level ofthe input signal is not low.

The bias control arrangement may be configured to provide to the secondand third amplifiers a pulse width modulated signal having a duty cyclethat is a function of the measured bias.

The apparatus may comprise an electrical connector, for instance a jackplug, for receiving an electrical input signal.

A third aspect provides a cable arrangement comprising first and seconddevices connected by a dual core optical waveguide, wherein the firstdevice comprises such apparatus.

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings, in which:

FIG. 1 is a schematic drawing of circuitry used to communicateelectrical audio signals in optical form, according to certain aspectsof the invention;

FIG. 2 is a schematic drawing of transmitter circuitry of FIG. 1,according to certain aspects of the invention;

FIG. 3 is a schematic drawing of an audio signal communication cableembodying the invention;

FIG. 4 is a schematic drawing of a guitar including the FIG. 2 circuitryand embodying the invention; and

FIG. 5 is a schematic drawing of a microphone including the FIG. 2circuitry and embodying the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference will firstly be made to FIG. 1, which is a schematic diagramillustrating components of a system 10. The system includes atransmitter 11, an optical cable 12 and a receiver 13.

In brief, the transmitter circuit 11 is arranged to receive electricalsignals at audio frequencies, to provide an amplitude modulated opticalsignal over first and second cores of an optical cable 12, and thereceiver 13 is arranged to demodulate the optical signals and toreproduce the original electrical signal. This specification isconcerned primarily with the transmitter circuit 11.

Firstly, the arrangement of components within the transmitter circuit 11will be described.

The transmitter circuit 11 includes first to fifth operationalamplifiers (op-amps) U1 to U5. Each of the op-amps has an invertinginput, a non-inverting input and an output.

The first op-amp U1 is an input amplifier, and acts as a buffer. Theinput amplifier U1 is connected to receive an input electrical signalsupplied at an INPUT terminal at its inverting input. The non-invertinginput of the first op-amp U1 is connected to a node in a chain betweenpositive and supply negative terminals, as is described in more detailbelow. Although not shown, buffering and filtering components areprovided to process a received signal before application to the INPUTterminal.

The chain includes, in sequence from the positive supply terminal:collector and emitter electrodes of a first transistor Q1, anode andcathode terminals of a first light source LED1, a tenth resistor R10, aneleventh resistor R11, a second light source LED2 and collector andemitter electrodes of a second transistor Q2. The first transistor Q1 isan NPN transistor, and the second transistor Q2 is a PNP transistor. Thenon-inverting input of the first amplifier U1 is connected at the nodebetween the tenth and eleventh resistors R10, R11. This node is alsoconnected to ground potential by a thirteenth resistor R13. The lightsources here are light emitting diodes.

The base electrode of the first transistor Q1 is connected to an outputof the second op-amp U2 by a ninth resistor R9. Similarly, the baseelectrode of the second transistor Q2 is connected to the output of thethird op-amp U3 by a twelfth resistor R12. The non-inverting inputs ofthe second and third op-amps U2 and U3 are connected to ground potentialGND. The inverting input of the second op-amp U2 is connected to theoutput of the first op-amp U1 by a fourth resistor R4. The invertinginput of the third op-amp U3 is connected to the output of the firstop-amp U1 by a seventh resistor R7. The inverting input of the secondop-amp U2 is also connected to the node between the first transistor Q1and the first light source LED1 by a fifth resistor R5. The invertinginput of the third op-amp U3 is connected to the node between the secondlight source LED2 and the second transistor Q2 by an eighth resistor R8.

A biasing arrangement is provided by the fourth and fifth op-amps U4 andU5.

The output of the fifth op-amp U5 is connected to the inverting input ofthe third op-amp U3 by a sixth resistor R6. The inverting input of thefifth op-amp U5 is connected directly to the output of the fifth op-amp,and is connected to the inverting input of the fourth op-amp U4 by asecond resistor R2. The first resistor R1 is connected between theoutput and inverting INPUT terminals of the fourth op-amp U4. The outputof the fourth op-amp U4 is also connected to the inverting input of thesecond op-amp U2 by a third resistor R3. The non-inverting input of thefourth op-amp U4 is connected to ground potential. The non-invertinginput of the fifth op-amp U5 is connected to receive a bias Voltagesignal VBIAS.

The first and second cores of the optical fibre cable arrangement 12 areconnected to respective ones of the first and second light sources LED1and LED2. The first and second cores are contained within a sheath,which is indicated schematically in the figure. At the receiver 13, thefirst core of the optical fibre 12 is connected to a first photodiodePD51 and the second core is connected to a second photodiode PD52.

The output signal from the first photodiode PD51 is amplified by a firstreceiver op-amp U51. A second receiver op-amp U52 is provided to amplifythe signal provided by the second photodiode PD52. Outputs of the firstand second receiver op-amps U51 and U52 are summed at a potentiometerVR51, and the result is amplified by a third receiver op-amp U53.

In the transmitter circuit 11, the first and second resistors R1 and R2may have the same value. Also, each of the third to eighth resistors R3to R8 may have the same value as one another. The ninth and twelfthresistors R9 and R12 may have the same value, which may for instance be100Ω. The tenth and eleventh resistors R10, R11 may have the same value,which may for instance be 10Ω. The thirteenth resistor R13 may also havea value of 10Ω.

Operation of the transmitter circuit 11 will now be described.

In brief, the second and third op-amps U2 and U3 drive the first andsecond light sources LED1 and LED2 respectively. The light sources aredriven based on the input signal received at the inverting input of thefirst op-amp U1. The second op-amp U2 drives the first light source 1 toprovide illumination at a level that is directly proportional to theinput Voltage for one half of the waveform of the input Voltage, and thethird op-amp U3 drives the second light source LED2 to provide anillumination at a level proportional to the Voltage of the input signalfor the other half of the waveform. It can be visualised that the firstlight source LED1 provides the positive part of the input signal and thesecond light source LED2 provides the negative part of the input signal.When the input signal is negative, the first light source LED1 providessubstantially no signal content, and when the input signal is positive,the second light source LED2 provides substantially no signal content.

Assuming that the value of a VBIAS applied to the non-inverting input ofthe fifth op-amp U5 is zero Volts, the outputs of the fourth and fifthop-amps U4 and U5 are also 0. When the input signal has a positivevalue, the output of the first op-amp U1 becomes more negative, which inturn causes the outputs of the second and third op-amps U2 and U3 tobecome more positive. This causes the current flowing in the first lightsource LED1 to increase and the current flowing in the second lightsource LED2 to decrease.

In turn, this causes the current flowing through the collector-emitterpath of the first transistor Q1 through the tenth resistor R10 and intothe thirteenth resistor R13 to increase. This causes a positive Voltageto appear across the thirteenth resistor R13. The action of the feedbackconnection from the thirteenth resistor R13 to the non-inverting inputof the first op-amp U1 results in the Voltage across the thirteenthresistor R13 following the input Voltage at the inverting input of thefirst op-amp U1. Since the Voltage across the thirteenth R13 isproportional to the current flowing though it and thus through theconducting one of the first and second light sources LED1, LED2 thecurrent through the conducting light source LED1 or LED2 is caused to beproportional to the Voltage at the input of the first op-amp U1. Assuch, this arrangement provides a feedback signal to the first op-amp U1that is proportional to the current in the light sources LED1 and LED2.

When the input signal is negative, a similar process results in currentflowing through the first resistor R1, the second light source LED2 andthe second resistor Q2, resulting in a negative Voltage across thethirteenth resistor R13.

With a value of VBIAS at zero Volts, the crossing of the signal suppliedat the INPUT terminal from positive to negative or from negative topositive would require the output of the second and third op-amps U2 andU3 to make large steps for the respective light source LED1 or LED2 tostart conduction.

Disregarding the effects of errors and offsets in the op-amps andassuming zero bias provided by the fourth and fifth op-amps U4 and U5,the output of the first op-amp U1 is zero if its input is zero, and theoutputs of the second and third op-amps are also 0. In this condition,the first and second transistors Q1 and Q2 are turned off, that is theyare non-conducting, and there is no current in either of the lightsources LED1 and LED2 and there is a zero Voltage across the thirteenthresistor R13. If even a very small input Voltage is applied at theinput, the feedback loop requires that one of the transistors Q1 and Q2be turned on to drive current through one of the light sources LED1 orLED2. This requires a Voltage of about +/−2 Volts at the output of thesecond or third op-amp U2 or U3 according to the polarity of the input.Typically, with real components, the outputs of the second and thirdop-amps U2 and U3 need only to change by a further 0.5 Volts to 0.7Volts for the maximum signal that the transmitter circuit 11 can handle.

The need for the provision of large steps in Voltage by the second andthird op-amps U2 and U3 is avoided by the biasing arrangement shown inFIG. 1 and shown in more detail in FIG. 2.

In particular, a bias current for the first and second light sourcesLED1 and LED2 is generated by the biasing circuit formed by the fourthand fifth op-amps U4 and U5. In particular, a Voltage approximatelyequal to the forward Voltage of the light sources LED1 and LED2 isapplied to the VBIAS input, which is the non-inverting input of thefifth op-amp U5. This causes the output of the fifth op-amp U5 to beequal to the Voltage of VBIAS and causes the output of the fourth op-ampU4 to be equal to the negative Voltage, i.e. −VBIAS.

Acting through the third and sixth resistors R3 and R6, the signals onthe outputs of the fourth and fifth op-amps U4 and U5 cause the thirdop-amp U3 to drive the base of the second transistor Q2 negative untilthe Voltage on the cathode of the second light source LED2 is −VBIAS andcause the output of the second op-amp U2 to be driven positive until theVoltage on the anode of the first light source LED1 is equal to VBIAS.

The use of two light sources driven by two different amplifiers togenerate signals on different phases of the input signal results in adoubling of the dynamic range, compared to the situation where a singlelight source is used. This advantage is achieved at the expense of anadditional core required for the optical cable 12. However, this isconsidered to be an acceptable downside considering the performanceadvantages that result from the increased dynamic range.

The present embodiment provides a Voltage adjustment arrangement bywhich it is possible to adjust the current flowing through the lightsources LED1 and LED2 in the absence of a signal at the input to a valuethat is suitable to reduce non-linearity whilst not consuming excessivepower.

Through the use of a bias Voltage adjustment arrangement, differences inforward Voltage for different light sources, resulting from differenttemperatures of operation and also from manufacturing tolerances, can beaccommodated.

In the embodiments described herein, this is achieved by measuring thebias current that flows through the light sources LED1 and LED2 and byadjusting the Voltage of the signal at VBIAS to maintain the biascurrent at the required level.

Moreover, the bias Voltage adjustment arrangement biases two separateamplifiers U2 and U3, and their associated light sources LED1 and LED2,in a balanced fashion using only one input VBIAS.

FIG. 2 shows the transmitter circuit arrangement 11 in more detail.Reference numerals are re-used from Figure for like elements. Theadditional elements that are shown in FIG. 2 but are not shown in FIG. 1will now be described.

The main additional component is a microprocessor μP. The microprocessorμP is configured to measure the bias current flowing through the lightsources and to provide a Voltage at VBIAS that is appropriate to achievethe required bias current level.

The microprocessor μP has two analogue inputs In1 and In2. The firstinput In1 is connected to sample the signal at the INPUT terminal. Thesecond input In2 of the microprocessor μP is connected to sample theVoltage at a node in the chain including the first and second lightsources LED1 and LED2.

In particular, the INPUT terminal is connected to an inverting input ofa seventh op-amp U7 via an eighteenth capacitor C18 and a twenty-fourthresistor R24. The output of the seventh op-amp U7 is connected to thefirst input In1 of the microprocessor μP by a twenty-fifth resistor R25.A twenty-sixth resistor R26 is connected between the inverting input andoutput terminals of the seventh op-amp U7. The non-inverting input ofthe seventh op-amp U7 is connected to the mid-point of a potentialdivider formed by twenty-seventh and twenty-eighth resistors R27 and R28connected between a second supply potential VDD and ground potentialGND.

The node between the cathode of the first light source LED1 and thetwelfth resistor R12 is connected to the non-inverting input of a sixthop-amp U6 by a thirtieth resistor R30. The inverting input of the sixthop-amp U6 is connected to ground potential GND by a thirty-secondresistor R32. A thirty-first resistor R31 is connected between theoutput and inverting INPUT terminals of the op-amp U6. The output of thesixth op-amp U6 is connected to the second input In2 of themicroprocessor μP by a thirty-third resistor R33.

An output Out of the microprocessor μP is connected to the non-invertinginput of the fifth op-amp U5 as VBIAS. The output Out of themicroprocessor μP is an analogue output. The output Out of themicroprocessor μP is connected in particular by an RC circuit comprisedof three resistors R19, R35 and R36 and two capacitors C6 and C20. TheRC circuit does not include any series capacitors, which would block DC,but includes a capacitor connected to ground potential GND.

Also, capacitors are connected across the outputs and inverting inputsof the second and third op-amps U2 and U3. These capacitors control thebandwidth of the amplifiers provided by the second and third op-amps U2and U3.

A signal of maximum amplitude approximately 1.2 Volts peak to peak ispresented at the INPUT terminal. Because the twelfth, thirteenth andfourteenth resistors R12 to R14 are all 10Ω resistors, the maximum peakcurrent through the first and second light sources LED1 and LED2 isabout 60 mA.

The microprocessor μP is configured to adjust the bias Voltage appliedto the non-inverting input of the fifth op-amp U5 by measuring the biascurrent as follows.

The bias current flowing through the light sources LED1 and LED2 resultsin a Voltage across the twelfth resistor R12. In the absence of anyVoltage on the INPUT terminal, the feedback loop from the node betweenthe twelfth and thirteenth resistors R12 and R13 to the non-invertinginput of the first op-amp U1 ensures that the Voltage at the nodebetween the resistors R12 and R13 is 0. As such, the Voltage at the nodebetween the twelfth resistor R12 and the first light source LED1 is afunction only of the bias current. The Voltage at this node is amplifiedby the sixth op-amp U6, having a gain determined by the values of thethirty-first and thirty-second resistors R31 and R32. The resultingamplified signal is proportional to the bias current. This signal isprovided to the second input In2 of the microprocessor μP. The secondinput In2 is an analogue is a digital converter (ADC) input.Consequently, the microprocessor μP is provided with digital values(data) that is indicative of the Voltage at the node between the twelfthresistors R12 and the first light source LED1, which is proportional tothe current flowing through the first light source LED1 in the absenceof a Voltage at the INPUT terminal.

The microprocessor μP is configured to sample the signal at the secondinput In2 at a high rate, for instance 1000 samples per second. Wheneverthe microprocessor μP determines that the level of the signal receivedat the second input In2 is sufficiently low that the signal inducedcurrent in one of the light sources LED1 and LED2 is below apredetermined threshold, the bias current is estimated. The bias currentis estimated by processing the digitised value resulting from theVoltage at the second input In2 of the microprocessor μP. The thresholdis chosen to have any suitable value. The threshold may be for instance20% of the desired bias current. A low threshold may result in the biascurrent not being adjusted for long periods of time in normal use, but ahigh threshold may result in the processor incorrectly estimating andadjusting the bias current due to signal induced currents in the twelfthresistor R12. A suitable threshold results in bias current adjustment atsuitably short intervals without providing sensitivity to signal inducedcurrents in the twelfth resistor R12.

The microprocessor μP is configured to filter the estimated bias currentvalues. The filtered estimates are used to drive a digital feedback loopthat is implemented in software in the microprocessor μP, an output ofwhich provides a digital value that represents the value of VBIAS thatis needed to provide the desired bias current through the light sourcesLED1 and LED2. In designing the software, an ideal bias current ischosen based on the need to extend battery life (which tends towards alow bias current) and achieving acceptable sound quality (based onmeasurements of signal distortion and listening tests). The softwareintegrates the estimated bias current measurement over time, taking newsamples when the signal is below the threshold, as discussed above, andadjusts the PWM output to increase or decrease the bias current so as tocause the integrated measurement to approach the ideal.

The value indicative of the desired level of VBIAS is used by themicroprocessor μP to provide at the output Out a pulse width modulated(PWM) signal having a duty cycle that is representative of the value ofVBIAS. The PWM signal is then filtered by the RC arrangement comprisingthe resistors R19, R35 and R36 and the capacitors C6 and C28 such as toprovide a substantially DC (direct current) signal to the non-invertinginput of the fifth op-amp U5.

The transmitter circuit provides particularly good noise performance, inlarge part because of the large peak LED current. The large peak LEDcurrent is possible because of the dual drive mechanism provided by theconnection of the second and third op-amps U2 and U3 as invertingamplifiers with feedback from the node in the path including the lightsources LED1 and LED2. This allows the current to be swung with a verylow bias current. Taking the example of swinging the current through 120mA, this can be achieved with about 5 mA of bias current. Achieving thesame with a class A circuit design would require 130 mA of bias current,which would present a very high draw on battery. Consequently a class Adesign would typically use a lower bias current, e.g. 40 mA biascurrent, which would reduces the peak signal (by about two thirds, or 10dB) which worsens noise by about 10 dB. The dual drive mechanism of thetransmitter circuit 11 reduces power consumption whilst increasing thesignal-to-noise ratio.

Moreover, in the transmitter circuit 11 bias voltages are injected froma single positive bias source, namely the output Out of themicroprocessor. This is possible because of the arrangement of thefourth and fifth op-amps U4 and U5, and is possible of the connection ofthe second and third op-amps U2 and U3 as inverting amplifiers withfeedback from the node in the path including the light sources LED1 andLED2. The microprocessor μP is configured to examine signals received atthe first input In1. The first input In1 is an ADC input, and themicroprocessor μP generates and processes digital values that arerepresentative of the analogue Voltage received at the first input In1.By virtue of the arrangement of the seventh op-amp U7, the analogueVoltage present at the first input In1 of the microprocessor μP isrepresentative of the Voltage at the INPUT terminal.

The microprocessor μP is configured to detect from signals received atthe first input In1 when there has been any significant signal receivedat the INPUT terminal. The microprocessor μP is configured to respond todetermining that no significant signal has been present at the INPUTterminal for a predetermined period of time by powering down thetransmitter circuit 11, in particular by powering down themicroprocessor μP and ceasing to provide supply Voltage to the Voltageterminals V+ and V−. The significant period of time may for instance be60 seconds, 300 seconds, 600 seconds etc.

In this way, the transmitter circuit 11 can experience reduced powerconsumption, because the transmitter circuit 11 is powered down and thusdoes not consume power when it is not in use. This can be convenient forthe user, who thus does not need to remember to turn off the transmittercircuit 11 after use. The user may turn the transmitter circuit 11 onthrough the use of a hardware switch (not shown).

The microprocessor μP may determine that an insignificant level ofsignal is present at the INPUT terminal if the level of signal does notexceed a threshold, or does not exceed a threshold for a thresholdperiod of time. The threshold may for instance be 0.5%, 1% or 2% of themaximum signal level, so 0.003, 0.006 or 0.012 Volts respectively, or inthe range of 0.5-2% of the maximum signal level. The threshold period oftime may for instance be a tenth of a second.

It will be appreciated that the above-described embodiments are merelyillustrative and are not limiting on the scope of the claims. Variousalternatives will be envisaged by the skilled person, and some will nowbe described.

Instead of using the microprocessor μP to measure the bias current andto provide the bias signal VBIAS, these functions could instead beperformed by a circuit of analogue components. Using a microprocessor isadvantageous in that relatively sophisticated processing can beimplemented relatively easily. Also, using a microprocessor as describedin the embodiments provides the additional advantage that themicroprocessor μP can also be used to determine when no significantinput signal is present and to power down the transmitter circuit 11accordingly. This function would be expensive to achieve using analoguecomponents.

Instead of using a Voltage at a terminal of one of the light sourcesLED1 and LED2 to allow measurement of the bias current through the lightsources, other arrangements are possible. For instance, in otherembodiments the Voltage across the tenth and eleventh resistors R10 andR11 are measured in order to measure the bias current. When the Voltageacross the tenth and eleventh resistors R10 and R11 are equal andopposite, the sum of their absolute magnitudes corresponds directly withthe bias current. The Voltages across the tenth and eleventh resistorsR10 and R11, and the calculation of the bias current, may be performedby a microprocessor, by analogue circuitry or by a mixture of analogueand digital circuits.

In the above embodiments, the signal at the second input In2 of themicroprocessor μP is sampled 1000 times per second. In otherembodiments, the signal of the second input In2 is sampled at a muchhigher frequency, in particular at a frequency that is significantlyhigher than the maximum frequency of the audio signals that are receivedat the INPUT terminal. In these embodiments, the bias current throughthe first and second light sources LED1 and LED2 can be measured and aVoltage for the signal VBIAS calculated every time that the input signalpasses through zero Volts. Better performance is achieved in theseembodiments, although at the cost of an increase in the processingrequirement of the microprocessor μP, which may require the use of ahigher capability microprocessor.

FIG. 3 shows a cable arrangement 30 according to various aspects of theinvention. A first jack includes a first metallic ¼ inch plug 31 and acorresponding body 32. Within the body of the first jack is a firstdevice comprising the transmitter circuit 11 and a 1.5 or 1.2V battery.The transmitter circuit 11 is connected to the first plug 31 so as to beable to receive analogue electrical audio frequency signals appliedthereto. The LEDs LED1 and LED2 of the transmitter circuit 11 in thefirst device are arranged so as to pass the light that it emits intoends of two cores of a shielded optical fibre 37, which comprises thecores Corel and Core 2 and the Sheath of FIG. 1. At an opposite end ofthe optical fibre 37, a second device 36 is included in a body of asecond jack, which includes a second ¼inch plug 34. The second device 36includes the receiver circuit 13 of FIG. 1 and a 1.5 or 1.2 V battery.The photodiodes PD51 and PD52 in the second device 36 are arranged toreceive light transmitted down the optical fibre 37. The electricaloutput of the second device 36 is provided to the second plug 34.

The cable arrangement 30 is used by mating the first plug into a signalout socket of a guitar or other sound source, and by mating the secondplug 34 into a signal in socket of a mixing desk or amplification deviceor similar. When the guitar is played, analogue electrical audiofrequency signals are provided to the conductors of the first plug, andthus are carried to the input of the transmitter circuit 11 included inthe first device. The first device thus amplitude modulates the audiosignal onto an optical signal at baseband, which is transmitted alongthe optical fibre 37. The baseband amplitude modulated optical signal isreceived at the second device 36, where the amplitude modulation isconverted into an electrical analogue audio frequency signal andprovided to the conductors of the second plug 34 and thus theamplification device or mixing desk that it is connected to. The signalsproduced by the audio source thus are carried to the amplificationdevice or mixing desk without being digitised and without beingtransmitted along a shielded electrical cable.

Manual distortion of the prototype optical cable 37 results in noaudible change in signal, since there is no signal degradation in theoptical cable 37.

Since current consumption of the FIG. 1 circuits is low, batteries areable to power the transmitter and receiver circuits 11, 13 correctly formany hours before requiring replacement.

The cable arrangement 30 suffers some disadvantages compared to theconventional electrical cable arrangement. In particular, a power supplyis needed at each end of the cable arrangement, whereas this is not trueof the electrical cable.

The two jacks are visibly different from one another, for instance bythe inclusion of arrows indicating the direction of signal transmission,or through the use of different patterning or colouring. This mitigatespotential problems from connecting the cable arrangement 30 the wrongway around (the cable arrangement 30 is unidirectional, and can carryaudio signals only from the first jack to the second jack). Theconventional electrical cable on the other hand is bidirectional.

A break in a core of the optical cable would normally result in theceasing of incorrect transmission, although such can also occur withconventional electrical cables.

Different audio sources produce different signal Voltages. For instance,an electric guitar produces an output signal having a maximum swing of 3or 4 Volts, whereas certain microphones produce only 50 mV or so.Although a cable arrangement optimised for use with an electrical guitarfunctions also with a microphone source, the converse is not true. Also,a cable arrangement capable of handling a relatively small signal swing,for instance 100 mV, is optimised for use with microphone sources and isbetter able to handle microphone-originating signals withoutdegradation. Accordingly, different cable arrangements, each optimisedfor a particular Voltage swing, may provide improved results than asingle, all-purpose arrangement. Different cable arrangements may beoptimised for different Voltage swings by the provision of differentbuffering and filtering arrangements prior to the INPUT terminal asshown in the circuit diagram of FIG. 1.

FIG. 4 illustrates schematically an electric guitar 40 according toaspects of the invention. The guitar includes three transducers, namelya neck humbucker 41, a middle coil 42 and a bridge humbucker 43. Each ofthese is connected to an electronic switching circuit 44, which includescontrollable potentiometers. The switching circuit 44 provideselectrical analogue signals at audio frequencies to a socket 47, withwhich a ¼ inch plug can be mated. The guitar thus far described isconventional.

The guitar 40 also includes a circuit 45, which comprises the FIG. 2transmitter circuit 11. The circuit 45 is connected to receiveelectrical analogue signals at audio frequencies from the switchingcircuit 44. The circuit 45 is powered by a power source, such as a 1.5or 1.2 V battery (not shown) included in the guitar 40. The circuit 45amplitude modulates the electrical analogue signal onto an opticalsignal and provides the result to an optical connector 46 mounted on aface of the housing of the guitar 40. A dual core optical cable (notshown) is connectable into the optical connector 46, and carries theoptical signal generated by the circuit 45 away from the guitar. Thus,the guitar 40 provides an optical amplitude modulated signal in the sameway that a combination of a conventional guitar and the first jack ofthe FIG. 3 cable arrangement 30 would provide. All of the benefitsstated above with relation to the previous Figures apply to thisembodiment. Since it includes an electrical signal output socket 47, theguitar 40 also is usable conventionally, although this can be omitted ifnot required.

FIG. 5 illustrates schematically a microphone 50 according to aspects ofthe invention. The microphone 50 includes a microphone transducer 51, asis conventional. The microphone transducer is connected to a circuit 52,which comprises the FIG. 1A or FIG. 1B circuit, the FIG. 3 transmitcircuit or similar. The circuit 52 is powered by a battery 53 Thecircuit 52 is connected to receive electrical analogue signals at audiofrequencies from the microphone transducer 51. The circuit 52 amplitudemodulates the electrical analogue signal onto an optical signal andprovides the result to an optical connector 54 mounted on a face of thehousing of the microphone 50. An optical cable 55 is removably orfixedly connected into the optical connector 54, and carries the opticalsignal generated by the circuit 52 away from the microphone 50. Thus,the microphone 59 provides an optical amplitude modulated signal in away similar to that of the FIG. 4 guitar 40 or the FIG. 2 transmittercircuit 11.

Although the above describes that the sound source can be a guitar ormicrophone, the sound source may be any other type that producesanalogue audio signals. The invention has most advantage with soundsources which are moved around during a performance, since these aremost likely to have electrical cables damaged during use.

An additional advantage is electrical isolation. Conventional cablingincludes electrical conductors. In the event of faulty amplificationequipment, electrical power could be transferred through the cable to aguitar player or other person, potentially resulting in electricalshock. The same could occur in the event of a lightning strike, whichmay occur at outdoors concerts and the like. Using an optical fibre toconvey audio signals, on the contrary, provides electrical isolationbetween the ends of the cable, and thus provides improved safety.

The human ear is able to perceive audio signals between 20 Hz and 20kHz, so it is normally only those signals that are of interest to amusician or sound producer. However, the carrying also of additionalsignal frequencies is not precluded by the invention, as long as thecontent of primary interest is in the audible frequency range.

Since the signal of interest is amplitude modulated onto the opticalsignal, the nature of the light source is not important, as long as theamplitude of its output is controllable. The light source need not be asingle frequency source, or even fixed in frequency. An LED makes a goodlight source because of its ease of use and low cost.

A circuit arrangement (not shown) may be added prior to the inputterminal so as to provide some frequency response shaping. Thearrangement may comprise plural RC circuits and a switch configured toswitch in one of the RC circuits and to switch out the others. This canallow a user to select a desired frequency shaping characteristic byselecting a suitable position for the switch. The arrangement may alsocomprise a buffer, for instance using an op-amp, between the true inputterminal and the RC circuit/switch arrangement.

The current driving of the LEDs LED1 and LED2 controlled by feedbackcontributes to good noise performance.

A prototype has been constructed using the arrangement 11 of FIG. 2.Performance was excellent, with a signal-to-noise ratio of 102 dB.Moreover, power consumption in the prototype is very low, giving rise toa long battery life. In tests, more than 12 hours use was achieved froman AA alkaline battery.

Although the invention has been discussed with respect to variousembodiments, it should be recognized that the invention comprises thenovel and non-obvious claims supported by this disclosure.

What is claimed is:
 1. Apparatus comprising: first and second lightsources; a first transistor having first and second main electrodes anda control electrode; a second transistor having first and second mainelectrodes and a control electrode; a first amplifier having first andsecond inputs and an output; a second amplifier having an input and anoutput; a third amplifier having an input and an output; and a biascontrol arrangement having an input and an output; wherein: the firstlight source and the first and second main electrodes of the firsttransistor are connected in series in a path between a positive supplyterminal and a first node, the second light source and the first andsecond main electrodes of the second transistor are connected in seriesin a path between the first node and a negative supply terminal, thefirst node is connected to a second input of the first amplifier, theinput of the second amplifier is connected to the output of the firstamplifier, the output of the second amplifier is connected to thecontrol input of the first amplifier, the input of the third amplifieris connected to the output of the first amplifier, the output of thethird amplifier is connected to the control input of the secondtransistor, the input of the first amplifier is connected to a signalinput terminal, the input of the bias control arrangement is connectedto a second node in the path through the first and second light sourcesand the first and second transistors, the bias control arrangement isconfigured to measure bias at its input and provide a control biassignal at its output, and the output of the bias control arrangement isconnected to the input of each of the second and third amplifiers. 2.Apparatus as claimed in claim 1, wherein the output of the bias controlarrangement is connected to the input of each of the second and thirdamplifiers via an arrangement comprising a fourth amplifier and abuffer.
 3. Apparatus as claimed in claim 2, wherein the output of thebias control arrangement is connected to an input of the buffer, whereinan output of the buffer is connected to the input of the third amplifierand to an input of the fourth amplifier, and wherein an output of thefourth amplifier is connected to the input of the second amplifier. 4.Apparatus as claimed in claim 1, comprising a first resistor connecteddirectly to the first node and in the path to the positive supplyterminal and a second resistor connected directly to the first node andin the path to the negative supply terminal, and wherein the second nodeis connected to the first node by either the first resistor or thesecond resistor.
 5. Apparatus as claimed in claim 1, wherein the biascontrol arrangement includes a microprocessor.
 6. Apparatus as claimedin claim 5, wherein the microprocessor is configured to monitor signalsreceived at the signal input terminal and is configured automatically topower down in response to detecting no significant input signal for aperiod of time.
 7. Apparatus as claimed in claim 1, wherein the biascontrol arrangement has a second input that is connected to the input ofthe first amplifier, and wherein the bias control arrangement isconfigured to calculate the bias control signal based on instances whenthe level of the signal at the second input is low and to refrain fromcalculating the bias control signal based on instances when the level ofthe signal at the second input is not low.
 8. Apparatus as claimed inclaim 1, wherein the bias control arrangement is configured to provideat its output a pulse width modulated signal having a duty cycle that isa function of the measured bias.
 9. Apparatus comprising: first andsecond light sources driven respectively by second and third invertingamplifiers with feedback from the first and second light sources toprovide signal content respectively on positive and negative phases ofan input signal; and a bias control arrangement configured to measure abias level of one of the light sources and to bias the second and thirdamplifiers based on the measured bias level.
 10. Apparatus as claimed inclaim 9, wherein the output of the bias control arrangement is connectedto the second and third amplifiers via an arrangement comprising afourth amplifier and a buffer.
 11. Apparatus as claimed in claim 10,wherein the output of the bias control arrangement is connected to aninput of the buffer, wherein an output of the buffer is connected to theinput of the third amplifier and to an input of the fourth amplifier,and wherein an output of the fourth amplifier is connected to the inputof the second amplifier.
 12. Apparatus as claimed in claim 9, whereinthe bias control arrangement includes a microprocessor.
 13. Apparatus asclaimed in claim 12, wherein the microprocessor is configured to monitorthe input signal terminal and is configured automatically to power downin response to detecting no significant input signal for a period oftime.
 14. Apparatus as claimed in claim 9, wherein the bias controlarrangement is configured to calculate a bias control signal provided tothe second and third amplifiers based on instances when the level of theinput signal is low and to refrain from calculating the bias controlsignal based on instances when the level of the input signal is not low.15. Apparatus as claimed in claim 9, wherein the bias controlarrangement is configured to provide to the second and third amplifiersa pulse width modulated signal having a duty cycle that is a function ofthe measured bias.
 16. Apparatus as claimed in claim 1, comprising anelectrical connector, for instance a jack plug, for receiving anelectrical input signal.
 17. Apparatus as claimed in claim 9, comprisingan electrical connector, for instance a jack plug, for receiving anelectrical input signal.
 18. A cable arrangement comprising first andsecond devices connected by a dual core optical waveguide, wherein thefirst device comprises apparatus as claimed in claim
 16. 19. A cablearrangement comprising first and second devices connected by a dual coreoptical waveguide, wherein the first device comprises apparatus asclaimed in claim 17.